In lithography for the manufacture of semiconductor components, scanners or steppers are used to project the structures of reticles, also known synonymously as masks, onto wafers coated with a light-sensitive layer, the resist. To produce ever-smaller structures on the wafer, it is necessary to increase the resolution with which the wafer is exposed. For example, in scanners for wafer exposure, the illumination is optimized according to the structures of the particular reticle to be imaged. Various illumination settings are used, which describe the intensity distribution of the illumination in a pupil plane of the illumination beam path of the scanner. Other methods of increasing the resolution include multiple illumination, for example, dual illumination, also commonly known as “double exposure,” and multiple structuring of the masks, for example, dual structuring, also commonly known as “double patterning.”
In these methods, the overall layer structure to be produced on the wafer is divided into two or more substructures formed on a set of different masks. In double exposure, a resist layer of the wafer is exposed with each substructure individually in succession. This is followed by the development of the resist and the etching of the wafer. In double patterning, after each substructure is exposed, the resist is developed, and, where appropriate, the wafer is etched and coated with new resist before the next substructure is exposed. Variants of double patterning are also known in which, for example, the resist is cured after exposure and another resist layer is then applied for further exposure.
An increase in resolution can be achieved in both methods by adjusting the illumination settings to the particular substructure. For example, an overall structure is divided into two substructures on two masks in such a way that each of the substructures contains grid structures composed of lines and spaces but rotated 90° from one substructure to the next. The aerial images of the substructures can then be captured using, as pupil filters, dipoles that are likewise rotated 90° from one substructure to the next. The dose and polarization of the exposure light are likewise adjusted to the particular substructure. To increase resolution in the case of line-and-space grid structures of one direction, the resolution limit can be increased by double patterning. Here, in the so-called double line process, every other line (or every other space, in the double trench process) is assigned to a different substructure. This has the effect of minimizing the achievable minimum spacing between lines (the pitch).
In the case of mask inspection microscopes, the structure of a reticle is projected onto a light-sensitive, spatially resolved image capture device, for example, a charge coupled device (CCD) chip. The image data are read by a computer and the data structure obtained is stored as a graphics file in the random access memory. The structure is magnified, for example by a factor of 450, so that any defects in the structure can be identified more precisely. Since the defects that are of interest in mask inspection are primarily those that will also show up on wafer exposure, the aerial images generated in the resist and on the detector should be as identical as possible, apart from the difference in imaging scale. To achieve equivalent image generation, the wavelength used during mask inspection, the illumination, and, at the object end, the numerical aperture are adapted to the scanner being used.
In double exposure and double patterning, the captured images of the substructures are overlaid in order to visualize the overall structure. A method of this kind is known from Patent Application DE10360536.